ABSTRACT The overall goal of this application is to elucidate the role of the death domain-associated protein (DAXX) in tumorigenesis. DAXX was initially identified by the PI as an adaptor protein interacting with the intracellular domain of the death receptor Fas (CD95/Apo-1). Subsequent studies by others and us have implicated DAXX in various scenarios of apoptosis, as well as a myriad of other cellular processes. Of note, pancreatic neuroendocrine tumors (PanNETs) frequently harbor mutations in DAXX and the DAXX-associated protein ATRX. These observations raise a critical issue of whether there is a defined biochemical activity that underlies the diverse function of DAXX in general and its tumor suppressive function in PanNETs in particular. Our preliminary data suggests that DAXX possesses a previously unanticipated activity in protein quality control (PQC). Here we plan to test the central hypothesis that DAXX is a novel catalyst for protein folding (foldase) and that the dysregulation of this activity contributes to the pathogenesis of PanNETs. We propose three specific aims. First, we will define the molecular mechanism of the protein folding catalyst activity of DAXX, and elucidate its regulation by interacting partners and post-translational modifications. Second, the tumor suppressor p53, which is inactivated in the vast majority of tumors, is rarely mutated in PanNETs. Based on our preliminary data, we will test the hypothesis that DAXX is a folding catalyst for p53, promoting its stability and functionality, and that the loss of DAXX contributes to the inactivation of this preeminent tumor suppressor in PanNETs. Third, both DAXX and the DAXX-interacting protein PML are involved in the alternative lengthening of telomeres (ALT) pathway, which maintains telomere length in various tumors including PanNETs. Our previous studies reveal an important role of PML in the clearance of misfolded proteins. We will examine the cooperation of DAXX and PML in the ALT pathway. Moreover, we will investigate the role of DAXX in the pathogenesis of PanNETs. Collectively, these aims will address critical issues in DAXX biology and will provide valuable information for the development of effective therapies for PanNETs and other DAXX-associated tumors.